1. Field of the Invention
The present invention relates to circuitry used to connect the analog input or output signals of a modem chip to a telephone line.
2. Description of the Related Art
Computer systems often need to communicate with each other by means of the telephone lines. Certain interfaces must be utilized to perform this communication. The telephone system uses analog signals, whereas the computer is a digital device. To this end a device called a modem is used to convert the computer digital signals from a series of b 1's and 0's into a series of analog tones which are then transmitted over the telephone lines to a matching modem connected to a second computer system. The second modem receives the analog tones and converts them to digital signals for transmission to the second computer.
Several problems arise in performing these translations of analog to digital and digital to analog. One is that the telephone interface is really a two wire system as implemented with full duplex capabilities, so that information is being transmitted and received over the same two wires at the same time. This is unacceptable for a digital computer system and so the modem must convert from two wires to four wires, which include receive, transmit, ring indication and ground signals. This four wire system can then be provided to a modem chip which converts the received analog signals into digital signals and provides transmitting analog signals from digital signals provided by the computer. The ring indicator output can go directly to the computer for its analysis.
Typically a transformer is used to help perform this translation to provide isolation and balancing needed for the telephone lines and to separate the various digital and power supply grounds from the telephone line interface. Conventionally a center tapped transformer is used in this application. In prior designs the center tap of the transformer was connected to ground while one leg of the computer side of the transformer was connected to a receive amplifier circuit and the other leg was connected to a transmit amplifier circuit. The problem that developed was that a 6 to 8 volt peak to peak signal had to be supplied by the transmit amplifier circuitry to properly drive the transformer to provide the necessary signal amplitude over the telephone lines for proper transmission and reception at the other end. When conventional operational amplifier circuitry is used, voltages greater than the peak to peak output voltages are necessary because of voltage drops contained in the operational amplifier. Therefore, for practical reasons 10 to 12 volts, either split or single supply, was required to power the transmit amplifier circuitry to properly produce the 6 to 8 volt peak to peak drive signal.
However because most parts of computer systems operated on 5 volt this need to develop 10 to 12 volts to allow the modem interface to couple to the telephone lines became a problem in that a separate power supply portion had to be used. Thus there was need to develop circuitry which allowed only a single 5 volt supply to power the modem interface circuitry used to couple the computer system to the telephone line.
Ring detect circuitry was also relatively complicated. Elaborate optocouplers, diode, capacitor and resistor networks were needed because of the high voltages present during ring and the need for isolation. The circuitry was elaborate and expensive.